Hitherto, for example, a Sn (tin)—Pb (lead) eutectic solder with a Pb (lead) content of, for example, 37% by weight has been widely used when electronic components are mounted on circuit boards. In recent years, however, the use of Pb-containing solder has been restricted from the viewpoint of achieving environmental protection, so that a Pb-free solder have been used.
A Sn—Ag—Cu alloy containing 3% by weight Ag (silver), 0.5% by weight Cu (copper), and the balance being Sn, is known as a typical Pb-free solder. Hereinafter, in the case of representing the composition of an alloy, the contents (wt %) are described before the symbols of elements. For example, the foregoing Sn—Ag—Cu alloy is represented by Sn-3 wt % Ag-0.5 wt % Cu.
In addition, a Sn-3.5 wt % Ag alloy and Sn-0.7 wt % Cu alloy are known as Pb-free solder alloys.
The following are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 62-252693    [Document 2] Japanese Laid-open Patent Publication No. 2001-334386    [Document 3] Japanese Laid-open Patent Publication No. 2010-167472